This invention relates to mixing graphics and video and, more particularly, to selective use of flicker filtering operations.
Motion video signals such as television programming may be combined with static signals generated by a processor-based system. The static signals are known as personal computer (PC) graphics signals, or simply graphics signals. The combined signal may be viewed on a television monitor.
A set-top box is a processor-based system that employs a television display instead of a computer monitor for viewing video signals, PC graphics signals, or a combination of the two. The set-top box may execute application software, such as electronic mail programs, connect to a data network, such as the Internet using browser software, and receive and display television programming signals.
Set-top boxes may combine a broadcast video signal with a graphics signal. The set-top box receives the video signal from an external source, such as via a coaxial cable, and mixes the signal with the PC graphics signal, typically generated from within the set-top box.
The set-top box may employ alpha values when combining the video signal with the PC graphic signal. Where a pixel, or picture element, results from the combination of a video element and a PC graphics element, an alpha value describes the weight of the elements, usually expressed as a percentage. The resulting pixel may be solid graphics, transparent graphics with video, solid video, and so on. The combining operation is known as alpha blending.
The television display receives video signals as a series of alternating fields: an odd field consisting of odd lines followed by an even field consisting of even lines, for example. The signal received by the television display is known as interlaced video. One odd field plus one even field constitutes a frame of the video signal.
One phenomenon of interlaced transmission is flickering. Flickering occurs when the video image includes small graphic detail, such as a single horizontal line, or other fine detail. For example, in an image including the letter “A”, the horizontal line in the letter may flicker as every other field is being displayed. Where the small detail (the horizontal line) appears in only one field, the line comes, then goes, as alternating odd and even fields are displayed.
Flicker filtering enhances graphics displayed on an interlaced monitor, such as a household television set. Flicker filtering blends pixels from scan lines of a field with adjacent scan lines from an associated, or opposite polarity, field. Thus, for example, where small graphic detail occupies a single scan line in an odd field, scan lines that would be displayed adjacent to the scan line in an even field may be updated to also include a weighted portion of the small graphic detail. By blending adjacent scan lines, the small detail image remains visible on the display, and flickering is lessened.
Flicker filtering may adversely affect motion video images, however. While the flicker filter generally may improve the appearance of the static graphics image, flicker filtering may also blur the detail of the video image. Further, flicker filtering allows images from two different instances of time (e.g., odd field, and even field) to “appear” on the display simultaneously. This tends to blur the image in time, particularly for moderately fast horizontal pans and action-laden scenes. Thus, although flicker filtering is beneficial for graphics images, it may diminish image quality for video images.
Thus, there is a continuing need to analyze the alpha blending value before performing flicker filtering operations.